Among variable-polarization-plane antennas provided with a right-hand circularly polarized antenna and a left-hand circularly polarized antenna, a configuration is known in which a phase shifter is connected to a feeder of one of the right-hand circularly polarized antenna and the left-hand circularly polarized antenna. Thereby, it is possible to change a polarization plane of a linearly polarized wave only by controlling a phase shift amount of the one phase shifter.
However, there is a problem that an amplitude difference between a right-hand and a left-hand circularly polarized waves occurs due to insertion loss of the phase shifter, and cross polarization discrimination deteriorates (a cross polarization component increases). Further, in order to maximize a variable range of the polarization plane of the linearly polarized wave (0 to) 180°, it is necessary to cause a variable range of the phase shift amount of the phase shifter to be 0 to 360°. That is, it is necessary to increase the variable range of the phase shift amount. A phase shifter the phase shift amount variable range of which is large has such problems that the phase shifter is physically large, that insertion loss is large, and that fluctuation of insertion loss caused by change in the phase shift amount and frequency is large.
On the other hand, a polarization plane control antenna is also known which controls a phase of one of a right-hand circularly polarized wave and a left-hand circularly polarized wave by a phase shifter and controls amplitudes of both waves by amplifiers, respectively. According to the polarization plane control antenna, it is possible to, by compensating fluctuation of insertion loss of the phase shifter by the amplifiers, the cross polarization discrimination can be improved.
However, since the insertion loss of the phase shifter differs according to frequency, it is necessary to adjust an amplitude according to frequency for transmission/reception, and there is a problem that configurations of each amplifier and the phase shifter are complicated. Further, it is difficult to use a wide frequency band while maintaining preferable cross polarization discrimination. In addition, there is also a problem that a waveform of a transmitted/received high-frequency signal is distorted by the variable amplifiers, a problem that harmonics occur, and the like.